Size conversion

A printed list of the exact metric sizes of imperial spanners, sockets & drill bits is handy. This is because some imperial tools make a good fit for metric nuts, bolts, etc, and vice versa (though of course many don't). A good example is 3/4", which is 19.05mm. A size conversion list can be stuck to the lids of socket, spanner & drill bit boxes.

Such a list

• saves needing to find a spanner/socket/drill bit already in use
• provides 2 of the same size tool for some sizes, which is occasionally very useful
• enables old imperial tools to replace missing metric ones
• avoids need to buy replacements sometimes
• enables some metric tools to be used on old imperial equipment.
• enables purchase & use of now very cheap old imperial tools
• effectively expands your metric drill bit collection to include fractional mm sizes

This is definitely not a recommendation to use the sizes of socket & spanner that don't fit well. Doing so can damage nuts & spanners, and injure hands when the tool slips off.

Size tables

AF

Most imperial sockets & spanners are measured AF, across flats. These are marked with the size, and no following letters. Conversion is a straightforward inch to mm conversion.

```  ____
/    \     AF means from
/      \    top to bottom
\      /    on this diagram
\____/
```

```64ths   Inch    mm      Fits

1/64            0.40
2/64  = 1/32    0.79
3/64            1.19
4/64  = 1/16    1.59    1.5
5/64            1.98    2
6/64  = 3/32    2.38
7/64            2.78
8/64  = 1/8     3.12    3
9/64            3.57
10/64 = 5/32    3.97    4
11/64           4.37
12/64 = 3/16    4.76
13/64           5.16
14/64 = 7/32    5.56    5.5
15/64           5.95
16/64 = 1/4     6.35
17/64           6.75
18/64 = 9/32    7.14    7
19/64           7.54
20/64 = 5/16    7.94    8
21/64           8.33
22/64 = 11/32   8.73
23/64           9.13
24/64 = 3/8     9.52    9.5
25/64           9.92
26/64 = 13/32   10.32
27/64           10.72
28/64 = 7/16    11.11   11
29/64           11.51
30/64 = 15/32   11.91
31/64           12.30
32/64 = 1/2     12.7
33/64           13.1
34/64 = 17/32   13.49
35/64           13.89
36/64 = 9/16    14.29
37/64           14.68
38/64 = 19/32   15.08   15
39/64           15.48
40/64 = 5/8     15.87
41/64           16.27
42/64 = 21/32   16.67
43/64           17.07
44/64 = 11/16   17.46   17.5
45/64           17.86
46/64 = 23/32   18.26
47/64           18.65
48/64 = 3/4     19.05   19
49/64           19.45
50/64 = 25/32   19.84
51/64           20.24
52/64 = 13/16   20.64
53/64           21.03
54/64 = 29/32   21.43
55/64           21.83
56/64 = 7/8     22.22   22
57/64           22.62
58/64 = 29/32   23.02
59/64           23.42
60/64 = 15/16   23.81
61/64           24.21
62/64 = 31/32   24.61
63/64           25.0
64/64 = 1"      25.4mm  25
```

Sizes are only listed under the 'fits' column where the imperial size is in general use (or was). 29/32" may be a good metric fit, but no-one has such size tools.

Whitworth

The Whitworth system dates from 1841, and was the first non-proprietary bolt size system to be widely adopted. It describes bolt head sizes and bolt threads. There are 4 whitworth bolt designations, known as:

• W (Whitworth)
• BSW (BS Whitworth)
• BSC (British Standard Cycle)
• BSF (British Standard Fine thread)

BS and BSW sizes are often seen on old tools. In some countries they're still in common use, and Whitworth is still used here for some niche applications.

Whitworth sizes are not Across Flats measurements.

```Whit     BSW/BSF   Inch AF   mm

1/16 W     -       0.256     6.90
3/32 W     -       0.297     7.54
1/8 W     3/16     0.340     8.64
3/16 W    1/4      0.445     11.30
1/4 W     5/16     0.525     13.34    (Camera tripod mounting)
5/16 W    3/8      0.600     15.24
3/8 W     7/16     0.710     18.03
7/16 W    1/2      0.820     20.83
1/2 W     9/16     0.920     23.37
9/16 W    5/8      1.010     25.65
5/8 W     11/16    1.100     27.94
11/16 W   3/4      1.200     30.48
3/4 W     7/8      1.300     33.02
13/16 W   15/16    1.390     35.31
7/8 W     1.       1.480     37.59
1. W      1.1/8    1.670     42.42
1.1/8 W   1.1/4    1.860     47.24
1.1/4 W   1.3/8    2.050     52.07
1.3/8 W   1.1/2    2.220     56.39
1.1/2 W   1.5/8    2.410     61.21
1.5/8 W   1.3/4    2.580     65.35
1.3/4 W   2.       2.760     70.10
1.7/8 W    -        -        76.70
2.1/4    3.150     80.01
2. W
2.1/2    3.550     90.17
2.3/4    3.890     98.81
3.       4.180     106.17
3.1/4    4.530     115.06
3.1/2    4.850     123.19
3.3/4    5.180     131.57
4.       5.550     140.97
4.1/2    6.380     162.05
```

BA

```BA       Inch AF   mm

8 BA     0.152     3.86
7 BA     0.172     4.37
6 BA     0.193     4.90
5 BA     0.220     5.59
4 BA     0.248     6.30
3 BA     0.282     7.16
2 BA     0.324     8.23
1 BA     0.365     9.27
0 BA     0.413     10.49  = 7/32 BS
```

Number & Letter

File:Us drill sizes 2.gif A few DIYers also have number & letter sizes.

```Gauge  Inch    mm

80	0.014	0.343
79	0.015	0.368
78	0.016	0.406
77	0.018	0.457
76	0.020	0.508
75	0.021	0.533
74	0.023	0.572
73	0.024	0.610
72	0.025	0.635
71	0.026	0.660
70	0.028	0.711
69	0.029	0.742
68	0.031	0.787
67	0.032	0.813
66	0.033	0.838
65	0.035	0.889
64	0.036	0.914
63	0.037	0.940
62	0.038	0.965
61	0.039	0.991
60	0.040	1.016
59	0.041	1.041
58	0.042	1.067
57	0.043	1.092
56	0.046	1.181
55	0.052	1.321
54	0.055	1.397
Gauge	inches	mm
53	0.059	1.511
52	0.064	1.613
51	0.067	1.702
50	0.070	1.778
49	0.073	1.854
48	0.076	1.930
47	0.079	1.994
46	0.081	2.057
45	0.082	2.083
44	0.086	2.184
43	0.089	2.261
42	0.094	2.375
41	0.096	2.438
40	0.098	2.489
39	0.099	2.527
38	0.101	2.578
37	0.104	2.642
36	0.106	2.705
35	0.110	2.794
34	0.111	2.819
33	0.113	2.870
32	0.116	2.946
31	0.120	3.048
30	0.129	3.264
29	0.136	3.454
28	0.141	3.569
27	0.144	3.658
Gauge	inches	mm
26	0.147	3.734
25	0.149	3.797
24	0.152	3.861
23	0.154	3.912
22	0.157	3.988
21	0.159	4.039
20	0.161	4.089
19	0.166	4.216
18	0.169	4.305
17	0.173	4.394
16	0.177	4.496
15	0.180	4.572
14	0.182	4.623
13	0.185	4.699
12	0.189	4.801
11	0.191	4.851
10	0.194	4.915
9	0.196	4.978
8	0.199	5.055
7	0.201	5.105
6	0.204	5.182
5	0.206	5.220
4	0.209	5.309
3	0.213	5.410
2	0.221	5.613
1	0.228	5.791
```
```Gauge	inches	mm

A	0.234	5.944
B	0.238	6.045
C	0.242	6.147
D	0.246	6.248
E	0.250	6.350
F	0.257	6.528
G	0.261	6.629
H	0.266	6.756
I	0.272	6.909
J	0.277	7.036
K	0.281	7.137
L	0.290	7.366
M	0.295	7.493
N	0.302	7.671
O	0.316	8.026
P	0.323	8.204
Q	0.332	8.433
R	0.339	8.611
S	0.348	8.839
T	0.358	9.093
U	0.368	9.347
V	0.377	9.576
W	0.386	9.804
X	0.397	10.08
Y	0.404	10.26
Z	0.413	10.49
```

What size is ok?

Sockets & spanners

It may be helpful to first explain that spanners and sockets aren't actually the size marked on them. Rather they're a size which will fit over nuts of the marked size. A slight degree of slack between tool and nut is required, otherwise spanners would often not fit on, and would often be an impractically stiff fit. So tools are already calculatedly oversize.

The question then is how much oversize is ok. And its not simple to answer, as it depends on several factors. There should be just a little free play between tool and nut. If the gap is too big, the tool can damage the nut and slip off under pressure. The acceptable gap also depends on the nut size, for example a difference of 1mm is ok on a 1" nut, but entirely non-functional on a 3mm nut. Steel nuts are more tolerant of oversize tools than brass. And the acceptable oversize also depends on the torque to be applied; at very low torque levels large amounts of oversize are ok. And finally it sometimes depends on whether the tool in use is hex, bi-hex or parallel jawed.

Perhaps someone will come up with an engineering table for safe gap limits some day. Until then I can only say that if there is any noticable amount of rotation of the tool relative to the nut, its no good, so the acceptable tolerances are fairly small.

The one exception to that is where the torque applied is very low. If a socket is to be rotated directly by hand, with no bar, then generally anything that even loosely fits will do.

Drill bits

With some jobs, drill bit size must be precise. But for most DIY this is far from true, and simply selecting the nearest size is usually absolutely fine. Thus more or less all imperial drill bit sizes are good for re-use in a metric world.

Size is critical when drilling a hole in metal that will be tapped, and close substitutes should never be used for this.

Labelling

If you want to use old tools as a regular part of a set of metric tools, remarking them in mm makes life easier.

Painting sizes on tends to wear off in use, it doesn't work well.

A good option is a die grinder fitted with any small abrasive tool, which can be used to write on metal like a pen.

Another option is to paint the whole outside of the spanner/socket/etc, and put a blob of that colour paint on the hole where it fits in the toolbox. (Each size uses a different colour paint.)

Issues

Some vehicles use a mixture of bolt systems by design. Land Rovers and some other old designs are known for this. Until 1955 Morris and MG used metric threaded bolts with their heads made to fit Whitworth sizes.